Modern digital computing equipment may use high-power, integrated circuit components mounted in dense configurations on narrowly spaced printed circuit boards placed inside compact equipment enclosures. Because of their fast speed, emitter-coupled logic (ECL) components are widely used in the industry. However, ECL components generate relatively large amounts of heat which can be as high as 65 watts per component. Heat sinks in thermal contact with such components are used to dissipate heat from the components.
Heat is normally convected from a heat sink by a cooling fluid such as air or water. Although air has a thermal conductivity and density which is lower than most fluids air cooling, is the simplest and least expensive method for removing heat from components. The cooling of high-power, integrated circuit components continues to offer a challenge for heat sink design, since the ability of heat sinks to disperse excess heat is a severe and practical design limitation for high performance circuits.
Factors which contribute to the cooling of a heat sink include: the thermal conductivity of the material used to construct the heat sink, the total surface area of the heat sink; the velocity of the cooling fluid and the fluid dynamic characteristics of the heat sink as it interacts with the flow of the fluid. The heat sink may be constructed with one or several cooling fins, in various configurations, mounted on a base in thermal contact with the circuit component. Increasing the height and number of cooling fins on a heat sink will increase the total surface area available for dissipating heat. However, increasing the height of the cooling fins and decreasing the spacing between the fins may create serious fluid dynamic problems with the heat sink.
The flow of a fluid near a substantially smooth surfaced plate is characterized by the growth of a thin, relatively slow moving, boundary layer. This boundary layer is created by the viscous drag of the stationary surface on the moving fluid. This boundary layer acts as an insulator that retards the convective heat-transfer process to faster moving and relatively cooler layers of the fluid further from the surface of the cooling fin.
In addition, the depth and narrowness of the troughs between the fins may create areas of flow stagnation that impede the flow of fluid at the center of the component precisely where the component is generally the hottest. This deterioration in heat transfer can be partially overcome by increasing the velocity of the fluid. There are two common systems used for cooling by air; high volume mass flow, where the velocity of the air is in the range of 2 to 4 meters per second, and low volume impingement, where the velocity of the air is in the range of 15 to 20 meters per second. In mass flow systems, air is generally blown parallel to the circuit board by fans mounted near the periphery of the enclosure. In impingement systems, high pressure air is directed perpendicularly at the heat sink through nozzles in a plenum plate placed in close proximity with the heat sink. With impingement systems the heat transfer rate can be improved by as much as a factor of two over conventional mass flow cooling. For example, some impingement air cooling systems dissipate heat at the rate of 65 watts per component or 4 watts/cm.sup.2 of package surface area.
However, the heat transfer rate with impingement cooling is approximately proportional to the fourth root of the air pressure at the nozzle exit point. For example, to increase heat dissipation per component from 4 to 6 watts/cm.sup.2 would require as much as a three fold increase in air pressure. Nozzle back pressure, acoustic emissions and enclosure design set practical limitations on the size of high pressure blowers used in computer systems.
U.S. patents attempting to overcome these limitations include: the use of complex fin combs (G. Chausson, U.S. Pat. No. 2,535,721, 1950); angular flanges on the cooling fins (R. T. Race, U.S. Pat. No. 2,984,774, 1961; L. Katz, U.S. Pat. No. 3,147,801, 1964; R. T. Schultz, U.S. Pat. No. 3,163,207, 1964); small fin segments in staggered arrangements (L. L. Marton, U.S. Pat. No. 3,421,578, 1969; P. G. Gabuzda, U.S. Pat. No. 4,682,651, 1987); angular flanges and baffles (F. W. Parrish, U.S. Pat. No. 3,435,891, 1969); circular pins (R. M. Moore, U.S. Pat. No. 4,541,004, 1985; R. C. Chu, U.S. Pat. No. 4,638,858, 1987); and a contoured base (A. J. Arnold, U.S. Pat. No. 4,823,869, 1989). None of these inventions consider the dynamics of the boundary layer the in cooling of the interface between the fluid and the surface. Understanding the interaction of the fluid and the surface is necessary to design a compact fluid dynamically efficient heat sink.